Abstract Risky sexual behaviors have high personal, social, and financial costs. While family communication about sex can reduce these behaviors, most studies focus on a single time point. However, early (10-13), middle (14-17) and late (18-21) adolescence each brings distinct developmental and relational processes. To be effective, conversations about sex need to fit with teens' developmental stage and experiences. Protective resources are particularly important during late adolescence as this period is characterized by sexual experimentation and risk-taking, and shows high levels of sexually transmitted diseases and unintended pregnancies. Few studies assess family sexuality communication during late adolescence, a period which often involves separation from parents but also growing mutuality with them, particularly for girls, which may create new opportunities for teen-parent communication. Various characteristics affect how a person perceives parent-teen conversations about sex: 1) teen gender, 2) the gender match between teen and parent, and 3) whether one is the parent or the teen. For example, parents share different messages with daughters than sons about sex, but the influence of teen gender can be confounded by the match in gender between the teen and parent, which is unexplored. Finally, parents and teens frequently view family sexuality communication differently, but what lies behind these differences is unexplored. Research is needed to guide parents as to whether and how to talk with their children about sex in ways that are relevant for and accessible to teens at different developmental stages, particularly late adolescence. To address NICHD's goals of identifying factors for preventing unintended pregnancy and reducing transmission of STDs, the proposed study provides a rare opportunity to longitudinally explore continuity and change in family sexuality communication over three critical developmental stages of adolescence (Aim 1). It will also compare how characteristics of teen gender, the gender match between the teen and parent, and teen and parent agreement across topics shape family communication about sex and relationships (Aim 2). To address these aims, this study uses a longitudinal qualitative design, extending existing qualitative data on teen-parent sexuality communication when teens were in 7th and 10th grade (N=23 parents and 23 teens) by conducting a third wave of interviews during late adolescence with the same sample of diverse families. Thematic analysis will be used to examine interview data. Given its qualitative focus, this study is largely exploratory. However, it is expected that female late adolescents will be more likely to talk with parents about sex than males and that higher teen-parent agreement in reported talk about sex will be associated with positive perceptions of family sexuality communication (Aim 2). This study is significant as the first in-depth, longitudinal examination of teen-parent sexuality communication over three key adolescent periods. Results will provide recommendations for parents, health providers, and educators on how to support teens' health by adapting sexuality communication across multiple stages of adolescence.